where we have inserted momentum basis representations of
\(S_{Q}^{\mu }\)  and \(S_{\bar{Q}}^{\mu }\)  that are given in analogy to eq. (REF ).
In case the normalization factors are to be included at the amplitude level,
we can use for their computation either the concrete 4-dimensional representations
of spinors and Dirac matrices, as listed for instance in [1]},
or employ the 4-dimensional spinor-helicity representation of
these objects [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}.
